(i) Unlike its, hers, ours, yours and theirs, the possessive of one is one's not ones.
(ii) One has a way of intruding in such a sentence as "The problem is not an easy one". "The problem is not easy" may be a neater way of saying what you mean.
(iii) What pronoun should be used with one? His or one's, for example? That depends on what sort of a one it is, whether "numeral" or "impersonal", to use Fowler's labels. Fowler illustrates the difference thus:
One hates his enemies and another forgives them (numeral) .
One hates one's enemies and loves one's friends (impersonal) .
(iv) "One of those who . . . " A common error in sentences of this sort is to use a singular verb instead of a plural, as though the antecedent of who were one and not those —to write for instance: "It is one of the exceptional cases that calls for (instead of call for) exceptional treatment".
(v) Avoid the construction "One of the most difficult, if not the most difficult, cases . . . " Neither case nor cases sounds right. Write "one of the most difficult cases, if not the most difficult".